Diesel engines are advantageous over gasoline engines in that diesel engines are much more fuel-efficient and generate much less carbon oxides than gasoline engines. But, diesel engines may generate, owing to their fuel composition and incomplete combustion due to engine strokes, gaseous waste products, hydrocarbons, carbon monoxide, and nitrogen oxides, which pose a serious health problem to the population at large.
In addition to these gaseous pollutants, diesel engines also emit “soot” particles comprising carbonaceous solids containing adsorbable hydrocarbons and inorganic compounds or very fine droplets of condensate or a agglomerates of “particulate matters”. The “particulate matter” referred to herein as “diesel soot” is particularly rich in condensed polynuclear hydrocarbons, some of which have been found to be carcinogenic. Owing to these factors, many countries have promulgated strict standards to minimize the discharge of diesel soot from automotive sources into the atmosphere.
For the last 50 years, many studies have been steadily and extensively carried out in order to solve the above problems involving diesel soot. A post-treatment method of removing such diesel soot which consists of capturing particulate matters with a ceramic filter and then oxidizing them has been considered as the most realistic method in practice. The most important feature of such post-treatment lies in designing the filter so that it can effectively capture the diesel soot as well as in developing a catalyst, which can completely oxidize the captured diesel soot at the temperature of the gas exhausted from the diesel engine.
For example, U.S. Pat. No. 4,759,918, to Homeier, et al., describes a catalytic composite, which contains Pt, Pd, or Rh as the catalytic metal and comprises a particulate filter coated with a sulfur-resistant refractory inorganic oxide selected from the group consisting of titania, zirconia, silica, silica-alumina and alumina, that has been treated to be sulfur-resistant, i.e. alumina treated with titania, zirconia, tungsten oxide, etc.
U.S. Pat. No. 5,658,546, to Kobayashi, et al., describes a denitration (deNOx) catalyst excellent in denitration (deNOx) activity and durability at high temperature, for catalytically reducing NOx in exhaust gas by using a reducing agent such as ammonia. This denitration (deNOx) catalyst is obtained by preparing a titanium-tungsten mixed oxide (component A) by co-precipitating a soluble titanium compound and a soluble tungsten compound under specific conditions, drying and calcining the co-precipitated mixed oxide, and then depositing at least one catalytic metal (component B) selected from the group consisting of cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium (Nd), nickel (Ni), and tin (Sn) on said titanium-tungsten mixed oxide.
U.S. Pat. No. 5,591,414, to Jacob, et al., describes a sorption-oxidation catalytic converter for the combined chemo-sorptive and oxidative cleaning of diesel engine exhaust gases, characterized in that the catalytic active materials consisting of a solid acid system V2O5/WO3/TiO2/SiO2/Al2O3 is doped with platinum in the form of an oxide. In this catalyst, sulfate is contained in an amount of less than 1%, and there is no description about the effect of the sulfate to the acidity and activity of the catalyst.
Rather, DE-A-42 29 471A, which is mentioned as prior art in said U.S. Pat. No. 5,591,414, describes that a titanium source can be selected from metatitanic acid, titanium sulfate, or sulfuric acid-treated titanium oxide and the difference of the titanium source does not give any difference in effects, and further the use of a sulfated titanium source cannot generate any improvement in the activity. Further, this German patent publication suggests that it is possible to use iron and vanadium in the form of their oxide or sulfates, and iron metal in its sulfate form has higher activity. However, there is no consistent tendency between the use of sulfate form and the catalytic activity, and it also fails to provide any reasonable explanation.
U.S. Pat. No. 5,911,961, to Horiuchi, et al., describes a catalyst for the purification of diesel engine exhaust gas, which uses Pt or Pd as the catalytic metal and titania or zirconia as the carrier, wherein the catalyst is a mixture of first catalyst particles and second catalyst particles, the first catalyst particles consisting of a catalytic metal and WOx carried on the first carrier particles and the second catalyst particles consisting of a catalytic metal carried on the second carrier particles.
U.S. Pat. No. 6,013,599, to Manson, describes a self-regenerating catalyst member, which may be used to remove particulate carbon and residual carbonaceous material from engine exhaust, wherein the catalyst contains at least one catalytic metal selected from a group consisting of metals of Group IB (e.g., copper), Group VIII (e.g., iron) and Group VB (e.g., vanadium). This patent describes that the use of a mixture of catalytic metal oxides may give several advantages.
Studies carried out so far show that a Pt/SiO2 catalyst has good activity because it can oxidize, in an indirect manner, the soot captured on a filter using nitrogen oxides, sulfurous acid gas, water, oxygen and the like contained in the exhaust gas. However, the platinum-silica catalyst system requires a high operation temperature of 400 to 600° C. Therefore, it is necessary to develop a new catalyst, which can completely remove the particulate matter at an operation temperature lower than 300° C.
Further, many other studies also have been carried out to develop catalysts for the purification of diesel engine exhaust gas, which use platinum as the catalytic metal. From these studies, it has been considered that titania carrier can produce better results than zirconia as the carrier material, and the higher acidity in the carrier can give the higher activity of the catalytic metal, that is, platinum [See, Am. Chem Soc. Symp. Ser., 552, 250 (1994)].
In this regard, U.S. Pat. Nos. 5,911,961 and 5,591,414 mentioned above also describe that catalysts containing a solid acid such as tungsten oxide or molybdenum oxide are improved in activity for removing nitrogenous compound and durability at high temperatures. However, they do not describe any relationship between the acidity of the carrier and the activity of the catalytic metal, and thus they provide no finding or suggestion of any other means to increase the acidity of the catalyst carrier.
Another reason that the acidity of the carrier can influence the activity of the catalytic metal lies in that there are aromatic compounds [PAH (Polycyclic Aromatic Hydrocarbon)] and particulate matters in the diesel engine exhaust gas. The aromatic compounds are of strong alkalinity, and the particulate matters have the same structure and properties as carbon black, which also have a strong alkalinity. It is believed that if the carrier has high acidity, PAH and the particulate matters can be easily adsorbed and oxidized on the carrier.
Methods to increase the acidity of the carrier material may include, for example, incorporation of a solid acid, use of sulfated titania, or the like. U.S. Pat. No. 5,591,414 describes the incorporation of a solid acid, such as WO3 or MoO3, and the use of sulfatized titania. However, the catalytic metal is vanadium, and platinum should be used in zero or a very small amount. Further, since the sulfate or sulfuric acid used in the preparation of titanium gel should be neutralized (to about pH 8) with ammonia, the amount of sulfate [SO42−] is very low, for example, less than 1% of the total weight of the catalyst. In addition, even if sulfated titania is present, the difference lies only in that the titanium precursor is used in a sulfate form instead of an oxide form. Although titanium sulfate is used as a precursor, theoretically, the acidity cannot increase or decrease. Thus, there is no statement in said document about the influence of the use of titanium sulfate on the acidity of the carrier prepared from titanium sulfate.
When sulfuric acid is added or impregnated directly into sulfur-resistant refractory oxide such as zirconia in order to increase the acidity of the catalyst, the sulfate or sulfuric acid on the surface of the zirconia can be easily detached by water or during the oxidation of hydrocarbon, thereby deteriorating the activity of the catalyst.
Further, the bonding between zirconium and platinum is stronger than that between titanium and platinum, When platinum as the catalytic metal is carried on a zirconia carrier, the activity on oxidation is low due to the strong bonding between platinum and zirconia.
The above mentioned problems make it more difficult to use sulfuric acid to increase the acidity of carriers comprising zirconia when platinum is used as the catalytic metal.
Under such circumstances, there has been no practical suggestion about a method to increase the acidity of catalyst or carrier, other than the use of a solid acid.
The inventor has extensively studied on the method to increase the acidity of a carrier by incorporating a solid acid such as tungsten oxide and/or sulfuric acid into the carrier when preparing a catalyst for the purification of diesel engine exhaust gases comprising at least one sulfur-resistant refractory oxide such as titania and at least one catalytic metal such as platinum or palladium.
Specifically, the inventor first prepared a catalyst composed of catalytic metal/tungsten oxide/titania by adding a tungsten oxide precursor and sulfuric acid to a titanium gel prepared from a titania precursor, shaping and calcining the resulted mixed gel at about 600 to 800° C. In such case, it was found that the more sulfuric acid was added, the higher acidity of the carrier was achieved to produce a catalyst metal with higher activity, that is, platinum or palladium. However, since the bonding force between the sulfate radical and the titanium atom is relatively weak, there arise problems in practice, in spite of the improvement in the acidity and activity.
However, it has been surprisingly found that, when zirconia or zirconia-based composite oxide, such as zirconia-titania composite oxide is employed as a carrier and tungsten oxide and sulfuric acid are added during the preparation of the carrier, the catalytic activity and durability of the catalyst are significantly improved without deterioration of the adhesion of the carrier particles to achieve excellent catalytic performance.